public IHttpActionResult PutLetter(int id, Letter letter)
        {
            if (!ModelState.IsValid)
            {
                return BadRequest(ModelState);
            }

            if (id != letter.Id)
            {
                return BadRequest();
            }

            db.Entry(letter).State = EntityState.Modified;

            try
            {
                db.SaveChanges();
            }
            catch (DbUpdateConcurrencyException)
            {
                if (!LetterExists(id))
                {
                    return NotFound();
                }
                else
                {
                    throw;
                }
            }

            return StatusCode(HttpStatusCode.NoContent);
        }
        public IHttpActionResult PostLetter(Letter letter)
        {
            if (!ModelState.IsValid)
            {
                return BadRequest(ModelState);
            }

            db.Letters.Add(letter);
            db.SaveChanges();

            return CreatedAtRoute("DefaultApi", new { id = letter.Id }, letter);
        }